1. Field of the Invention
The present invention relates to a wavelength tunable light source and a pulse light source, and more particularly to a wavelength tunable light source capable of selectively generating light with different wavelength and a pulse light source capable of generating multiple-wavelength pulse trains simultaneously. The wavelength tunable light source and pulse light source are used as a light source of an optical path routing system called an optical routing system, an optical signal processing system, and a wavelength multiplexing transmission system, as well as a light source of a spectroscope for investigating physical properties.
2. Description of the Related Art
There are a number of reports about the wavelength tunable light source capable of varying the operational wavelength artificially.
For example, Japanese Patent Application Laid-open No. 8-146474 (1996) discloses a device capable of generating a short, high intensity optical pulse at low noise. The device is configured such that it generates a short optical pulse by a light source, amplifies by an optical amplifier the optical intensity of the optical pulse to an optical intensity level beyond the level capable of generating an optical soliton, and inputs the amplified optical pulse to an optical fiber with an anomalous dispersion characteristic to form an optical soliton, and that it shifts the optical soliton from the wavelengths of the optical noise involved in the amplification by the self-frequency shift effect of the optical soliton, and blocks the wavelength components of the optical noise in the output light from the optical fiber by an optical filter, thereby generating a short, high intensity, low noise optical pulse.
Japanese Patent Application Laid-open No. 2000-105394 discloses a compact wavelength tunable short optical pulse generator capable of tuning the wavelength without adjusting its optical system, and generating an ideal femtosecond soliton pulse. The device comprises a short pulse light source, an optical characteristic regulator for regulating the characteristics of the light supplied from the short pulse light source, and an optical fiber that receives an input pulse from the optical characteristic regulator and linearly varies the wavelength of the output pulse. With such a configuration, it can launch the short optical pulse into the optical fiber, generate a soliton pulse utilizing the nonlinear effect in the optical fiber, and shift the wavelength of the soliton pulse linearly with respect to the incident optical intensity by the nonlinear effect.
Furthermore, Japanese Patent Application Laid-open No. 2000-258809 discloses a multiple wavelength light pulse generating system capable of generating ultra-short optical pulse trains with a plurality of wavelengths simultaneously using a wavelength tunable pulse light source as its light source. This system comprises an optical demultiplexer for demultiplexing a short optical pulse output from a femtosecond fiber laser into a plurality of pulses, and a plurality of optical fibers for guiding the optical pulses output from the optical demultiplexer.
Although these types of the wavelength tunable light sources are inadequate in their stability and operability, there are various types of wavelength tunable light sources that can cover a wide wavelength range from the vacuum ultraviolet to extremely high frequency region. For example, as wavelength tunable light sources used in the optical communications in the near infrared and the more extended infrared region, there are optical parametric oscillators and Raman lasers utilizing optical nonlinear effect. Besides these light sources, dye lasers, solid state lasers and semiconductor lasers (called LD (Laser Diode) from now on) are available as the wavelength tunable light sources.
The optical parametric oscillator is configured such that a nonlinear optical crystal capable of generating frequency components different from the frequency of the pumping light is inserted into an optical cavity composed of a plurality of reflecting mirrors, and oscillates by pumping it with suitable pumping light. Selecting the optical nonlinear crystal and pumping light appropriately enables it to vary the wavelength in a wide range from 200 nm to 1,600 nm band, and even up to 5,000 nm to 70,000 nm.
The Raman laser is a light source that utilizes the amplification phenomenon of the Raman scattering light by irradiating a material with strong pumping light, and generates coherent light with a frequency of the Stokes ray or anti-Stokes ray. In particular, the Raman laser utilizing high-order Raman shift has a wide range wavelength tunable characteristic. It is reported that selecting appropriate nonlinear medium and pumping light enables it to vary the wavelength from the 500 nm band to 50,000 nm band.
The short optical pulse dye laser is a light source that achieves fluorescence by exciting liquid that dissolves an organic dye by a solvent. It is known that selecting the type of the dye and the wavelength of the pumping light makes it possible to tune the wavelength in the wavelength band from 300 to 900 nm.
As the solid state laser, a titanium sapphire laser is known which has a gain band region in a 680 to 1,100 nm range by absorbing light of 400 to 600 nm. The titanium sapphire laser utilizing a nonlinear optical crystal such as BBO (β-BaB2O4) or LBO (LiB3O5) has a tunable range of 300 to 1,100 nm band. It is reported that the BBO can vary the wavelength in a wide band from 1,100 to 10,000 nm by inputting 1,100 nm high output light.
The LD can achieve the wavelength tunable characteristic over 1,200-1,600 nm band by combining it with an external cavity and by carrying out chip selection and temperature control appropriately. It has an output characteristic above a few milliwatt output, and is characterized by wavelength stability, compact body size and high operability. Accordingly, it is utilized by wide spectrum of users from a research field to a practical field.
It is essential for the communications light source to be compact and stable. Accordingly, the dye laser, optical parametric oscillator and solid state laser are difficult to be applied to the communication light source. This is because the dye laser is unstable in the oscillation because of the liquid laser medium, and the other two lasers are composed of spatial optical system vulnerable to vibration and dust. Consequently, the light sources other than the LD are not actually used in communications at present.
In addition, the communication wavelength tunable light source must meet the following conditions: a short line width (spectral width) and sufficient output power above a few milliwatts; a tunable width capable of covering the bandwidth of optical amplifiers such as EDFAs (Erbium Doped Fiber Amplifiers) from several hundred to several thousand gigahertz band; and stable operation at desired frequencies. As for the desired frequencies, the ITU-T recommendation defines that the reference (anchor) frequency is 193.1 THz (1,552.524 nm), and the frequency spacing (frequency grid) is 100 GHz (about 0.8 nm) or 50 GHz. Accordingly, the wavelength tunable light source must operate at the wavelength of 1,552.524 nm±0.8 M or 1,552.524 nm±0.4 M, where M is an integer. The frequency stabilization on the order of {fraction (1/10)} of the channel spacing, that is, the accuracy of about 10 GHz (about 0.08 nm) or 5 GHz (about 0.04 nm) is required.
The external cavity LD, which provides an output with a short line width and above a few milliwatts, has the performance suitable for practical use as the light source of the communication. The external cavity LD, however, must perform mechanical cavity control for achieving wavelength tunability. Accordingly, it has a slow tunable rate of about milliseconds to be used as the wavelength tunable light source, thereby offering a problem of having non-oscillation frequencies (mode hopping). In addition, the tunable wavelength bandwidth is limited to several tens to one hundred nanometers for a single LD chip. Thus, the wavelength tunable light source using the external cavity LD has problems in the tunable rate, operation stability and tunable bandwidth.
As for the Raman laser that has a wide wavelength tunable bandwidth, its tunable frequency is determined by the material characteristics of the nonlinear medium controlling the amplification of the Raman scattering light. Using a silica optical fiber as the nonlinear medium gives a large Raman frequency shift amount of about 100 nm. Accordingly, the tunable frequency is limited, and the operation at a desired frequency is difficult. In addition, since the threshold of producing the stimulated Raman scattering is usually high, a problem arises in that the pumping light requires high output power laser.